Anti-seismic device

ABSTRACT

Anti-seismic device for seismically isolating structure relative to ground including first support defining first support plane integrally connectable to upper portion and including two first hinges defining first constant reciprocal distance, second support defining second support plane including two second hinges defining second constant reciprocal distance, third support defining third support plane integrally connectable to lower portion and including two third hinges defining third constant reciprocal distance, and connector defining connection plane perpendicular to third support plane and including at least two first rigid bars, each defining a first non-deformable connection direction and two second rigid bars, each defining a second non-deformable connection direction. First bars transiently constrained to first hinges and second hinge so the first connection directions of first bars cross in the connection plane. Second bars transiently constrained to second hinges and third hinge so second connection directions of second bars cross in connection plane.

The present invention relates to an anti-seismic device of the typespecified in the preamble of the first claim.

In particular, the present invention relates to an anti-seismic jointadapted to absorb vibrations for building type structures andinfrastructures in order to stabilize said structures in the presence ofseismic vibrational phenomena.

As is known, a plurality of anti-seismic solutions are currently used atthe construction level, also regulated by the regulations in force ineach country.

At the regulatory level, for example, there are buildings characterizedby a hyperstatic structure with regularity in plan and height, i.e.developing a compact and symmetrical plan and in which all the resistantvertical systems, such as frames and walls, extend throughout the heightof the construction.

In addition, the masonry elements comprise metal cores that allow thestructure of the building to have a predetermined deformability beforereaching catastrophic collapse.

In addition, national regulations often specify that a single type offoundation should be adopted for a given elevated structure, unless itconsists of independent units. In particular, the simultaneous use ofpile or mixed foundations with surface foundations must be avoided inthe same structure.

To ensure that the structure can resist, without major damage, seismicactivities, even quite intense, seismic insulators can be used.

These are positioned between the foundations and the structures inelevation to decouple the frequencies of the earthquake from thefrequencies of the structure in elevation and avoid the onset ofresonance phenomena. Using seismic insulators, the structure remainselastic even during violent earthquakes and preserves the dissipativeenergy capacities offered by ductility.

An example of a seismic insulator is the LRB or Lead Rubber Bearingswith a lead core consisting of alternate layers of steel and elastomerconnected by vulcanisation which, thanks to its high dissipativecapacity, can reduce horizontal displacement.

The energy dissipation provided by the lead core, through itsplasticization, permits an equivalent viscous damping coefficient to beobtained of up to about 30%. Thanks to the high dissipative capacity, itis possible to reduce the horizontal displacement, compared to that ofan insulation system with the same equivalent stiffness but with lessdissipative capacity.

These are usually circular, but can also be made with a squarecross-section, possibly with more than one lead core.

They are used on buildings, bridges or other structures, duringconstruction or seismic adaptation. They guarantee the safety of thestructure and what it contains Another type of insulator is provided bybuckling-restrained axial hysteretic dissipaters for example of theBRAD® series (Buckling-Restrained Axial Dampers). These are non-linearseismic devices the behaviour of which depends essentially ondisplacement. They are particularly suitable for use as dissipativebraces, for seismic protection by energy dissipation, and in particularfor seismic adaptation, of steel frame buildings. The insertion of thesedevices within the structural meshes increases the dissipative capacityof the structure, and therefore significantly improves its response tothe earthquake. Until yield is reached, BRAD® dissipators increase thestiffness of the structure, an effect that can be particularly usefulfor compliance with the regulatory requirements for limitinginter-storey movement to the Limit of Damage State, i.e. breaks allowedto the structure according to the safety margins in force.

The prior art described has several significant drawbacks.

In particular, the systems described, especially in the case of LeadRubber Bearings, are characterized by extremely complex structuresadapted to dissipate at least part of the deformation energy resultingfrom seismic phenomena.

These structures are therefore very onerous, in terms of cost, and makeit possible to address the problem of the management of seismicvibrations only in terms of damage tolerance, i.e. tolerance of damagewithin the Damage Limit State, resulting from deformation phenomenasometimes even plastic.

As a result, the systems of the previous type described are reactive andirreversible beyond certain earthquake thresholds.

In fact, all the devices known to the current state of the art work onlyon the stiffness of the joints and support structures.

In this situation the technical purpose of the present invention is todevise an anti-seismic device able to substantially overcome at leastsome of the drawbacks mentioned.

Within the scope of said technical task it is an important object of theinvention to obtain an anti-seismic device that is capable ofseismically isolating the foundations of a building or a supportstructure from the ground during, for example, a seismic vibratoryactivity, limiting the deformations of the device.

Another important object of the invention is to make an anti-seismicdevice that is capable of seismically isolating a structure without onlyintervening on the stiffness of the support joints of said structure.

In conclusion, a further object of the invention is to realize anisolation device that makes it possible to reduce the degrees of freedomof movement to which the structure that is supported by the ground issubjected with respect to the original reference system of saidstructure.

The technical purpose and specified aims are achieved by an anti-seismicdevice as claimed in the appended claim 1.

Preferred technical embodiments are described in the dependent claims.

The characteristics and advantages of the invention are clearly evidentfrom the following detailed description of preferred embodimentsthereof, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic model of a device according to the invention infree condition;

FIG. 2 illustrates a schematic model of a device according to theinvention subjected to seismic stress;

FIG. 3 is an embodiment example of a device according to the inventionin free condition;

FIG. 4 represents an embodiment of a device according to the inventionsubjected to seismic stress;

FIG. 5 shows an example of a foundation comprising two devices accordingto the invention arranged in a coplanar manner;

FIG. 6 illustrates an example of a foundation comprising fouroverlapping devices according to the invention;

FIG. 7a represents a second embodiment of a device according to theinvention in a first configuration;

FIG. 7b represents a second embodiment of a device according to theinvention in a second configuration; and

FIG. 7c represents a second embodiment of a device according to theinvention in a third configuration.

Herein, the measures, values, shapes and geometric references (such asperpendicularity and parallelism), when used with words like “about” orother similar terms such as “approximately” or “substantially”, are tobe understood as except for measurement errors or inaccuracies due toproduction and/or manufacturing errors and, above all, except for aslight divergence from the value, measure, shape or geometric referencewhich it is associated with. For example, said terms, if associated witha value, preferably indicate a divergence of not more than 10% of saidvalue.

In addition, where used terms such as “first”, “second”, “upper”,“lower”, “main” and “secondary” do not necessarily refer to an order, apriority relationship or relative position, but may simply be used tomore clearly distinguish different components from each other.

The measurements and data presented herein are to be considered, unlessotherwise indicated, as made in Standard International Atmospheres ICAO(ISO 2533).

With reference to the Drawings, reference numeral 1 globally denotes theanti-seismic device according to the invention.

The seismic device 1 is preferably adapted to seismically isolate astructure 2 with respect to the ground 3.

The structure 2 is preferably a building type structure. Therefore itcan be a building, an infrastructure such as a bridge or other type.

In addition, the term “structure” 2 may be understood not only as thestructure as a whole, but also as a portion of the structure.

The device 1 can in fact be housed in the foundations of the structures2, or it can be arranged in intermediate portions thereof. In oneexample, the device 1 is arranged at the base of the foundations of aresidential building, i.e. a house. In a second example, the device 1 isarranged below a bridge support pylon.

In a third example, the device 1 may be housed in the bridge portioncomprising the coupling between the support pylon and the transitcarriageway of the bridge itself.

The ground 3 may be a bottom of any type, preferably flat.

The ground 3 may be, for example, solid earth or a seabed.

In general, the device 1 is connectable to an upper portion and a lowerportion.

The lower portion may consist of the ground 3. However, it need notnecessarily be the ground 3, but may consist of other.

Similarly, the upper portion may consist of the structure 2, but doesnot necessarily consist of it.

As already mentioned above, in fact, the device 1 can assume differentconfigurations that include, for example, the arrangement withinintermediate zones of said structures 2.

The device 1 is described below in structural terms considering theconstituent parts thereof following the modelling of constructionscience. This means that, for example, when reference is made to hingesand bars, they refer to physical elements that exhibit behaviour similarto a bar and/or a hinge, in particular in a two-dimensional plane, butwithout any limitation with regard to the actual physical componentsused.

For example, a hinge may be made by a plurality of joints, just as arod, in terms of modelling, may refer to a bar, a beam or other elementsadapted, in this case, to connect hinges or otherwise having its ownstiffness.

The support 1 preferably comprises a first support 4, a second support 5and a third support 6.

The first support 4, the second support 5 and the third support 6preferably define similar forms.

Preferably the first support 4 defines a first support plane 4 a.

The first support 4 is preferably connectable to the upper portion, e.g.to the structure 2, or, in another example, to the third support 6 of anadditional device 1.

Thus, the first support plane 4 a may consist of the interaction orconstraint plane between the first support 4 and the structure 2.

In addition, the first support 4 comprises at least two first hinges 40.

The hinges 40, preferably, are made of mechanical joints that allowother elements to be transiently connected. Such mechanical joints maybe bolts adapted to preferably allow only a degree of transience, inparticular rotation around the hinge, of the other elements.

Such first hinges 40 are further preferably mutually spaced defining afirst distance d′.

The first distance d′ is preferably defined along the first supportplane 4 a.

In addition, it is preferably constant, so the first support 4 defines arigid rod.

Preferably the third support 6 defines a third support plane 6 a.

The third support 6 is preferably connectable to the lower portion, e.g.to the ground 3 or to the first support 2 of a second device 1.

The term “lower” as well as the term “upper” used previously, is definedwith reference to the ground 3 along the vertical direction defined, forexample, by gravitation acceleration.

Consequently, the third support plane 6 a may consist of the interactionor constraint plane between the third support 6 and the ground 3.

Further, the third support 6 comprises at least two third hinges 60.

Also the third hinges 60, preferably, are made of mechanical joints thatallow other elements to be transiently connected. Such mechanical jointsmay be bolts adapted to preferably allow only a degree of transience, inparticular rotation around the hinge, of the other elements.

Such third hinges 60 are further preferably mutually spaced defining athird distance d′″.

The third distance d′″ is preferably defined along the third supportplane 6 a. Further, it is preferably constant, so the third support 6defines a rigid rod.

In addition, preferably, the distance d′″ is congruent with respect tothe first distance d′. Alternatively, in the example of FIGS. 7a-7c thefirst distance d′ is greater, preferably by a percentage in the range of18% to 25% and more preferably in the range of 21 to 23%, compared tothe third distance d′″.

Preferably the second support 5 defines a second support plane 5 a.

The second support 5 is preferably connectable to the first support 4and to the third support 6.

As a result, the first support plane 5 a is comprised between the firstsupport plane 4 a and the third support plane 6 a.

In addition, the second support 5 comprises at least two second hinges50. Preferably, in the example of FIGS. 7a-7c the second support 5comprises four second hinges 50, two second upper hinges 50 a and twosecond lower hinges 50 b. The second hinges 50, preferably, are made,like the others, from mechanical joints that allow other elements to betransiently connected. Such mechanical joints may be bolts adapted topreferably allow only a degree of transience, in particular rotationaround the hinge, of the other elements.

Such second hinges 50 are, moreover, preferably mutually spaced defininga second distance d″. Preferably, in the example of FIGS. 7a-7c thesecond support 5 defines a second lower distance d₁″, between saidsecond lower hinges 50 b, and a second upper distance d₂″, between saidsecond upper hinges 50 a.

The second distance d″ is preferably defined along the second supportplane 5 a. In addition, it is preferably constant, so the second support5 defines a rigid rod. Preferably, the second distance d″ is notcongruent with the first and third distance d′ and d′″ but is less thanthem.

For example, the second distance d″ can be, compared to the thirddistance d′″ at least 3%, more appropriately 5% less.

Alternatively, in the example of FIGS. 7a-7c the second lower distanced₁″ is less than both said first and said third distance d′ and d′″, bya percentage preferably in the range of 40% to 50% and more preferablyin the range of 44 to 48%, relative to the first distance d′.Furthermore, the second upper distance d₂″ is greater than both saidfirst and said third distance d′ and d′″, by a percentage preferablybetween 9% and 15% and more preferably between 11 and 13%, compared tothe third distance d′″.

The device 1 then comprises connection means 7.

The connection means 7 are preferably adapted to connect the support 4,5, 6. They preferably define a connection plane 7 a. The connectionplane 7 a is perpendicular to the third support plane 6 a. Therefore, itis substantially perpendicular to the ground 3 and vertically joins,with respect to the ground, the supports 4, 5, 6.

The connection means 7 comprise at least two first bars 70 and twosecond bars 71.

The first bars 70 are preferably substantially rigid. In addition, theyeach define a first connection direction 70 a.

The first connection direction 70 a corresponds to the main extensiondimension of the bar 70 and therefore corresponds, for it, to the axialdirection.

The first connection direction 70 a is also non-deformable.

Preferably, the first bars 70 are adapted to constrain the first support4 and the second support 5.

More specifically—the two first bars 70 are each transiently constrainedto a first hinge 40 and a second hinge 50 so that the directions ofconnection 70 a of the first bars 70 cross in the connection plane 7 a.

In the example of FIGS. 7a-7c the first bars 70 are transiently attachedeach to a first hinge 40 and to a second upper hinge 50 a andsubstantially define the same geometry described.

Likewise, preferably, the second bars 71 are also rigid. In addition,they each define a second connection direction 71 a.

The second connection direction 71 a corresponds to the main extensiondimension of the bar 71 and therefore corresponds, for it, to the axialdirection. The second connection direction 71 a is also non-deformable.

Preferably, the second bars 71 are adapted to constrain the secondsupport 5 and the third support 6.

More specifically, the two second bars 71 are each transientlyconstrained to a second hinge 50 and to a third hinge 60 so that thesecond directions of connection 71 a of the second bars 71 cross in theconnection plane 7 a.

In the example of FIGS. 7a-7c the second bars 71 are transiently eachconstrained to a second lower hinge 50 b and to a third hinge 60 andsubstantially define the same geometry described.

The first bars 70 and the second bars 71 are preferably congruent witheach other, but may also be different.

The device 1 thus defines, substantially, preferably two similaroverlapping, consequential and mirror-like structures, at least in afree condition, with respect to the second support plane 5 a.

These structures are given by the first support 4, first bars 70 andsecond support 5 and second support 5, second bars 71 and third support6.

In the example of FIGS. 7a-7c the distance, in the vertical directionand in an aligned configuration (FIG. 7a ), between the first hinges 40and the second upper hinges 50 a is preferably very close to the firstdistance d′, and differs from the same preferably by less than 3%,preferably less than 1%. In addition, the distance, in a verticaldirection and in the aligned configuration (FIG. 7a ), between thesecond lower hinges 50 b and the third hinge 60 is preferably greaterthan the third distance d′″, by a percentage preferably between 12% and20% and more preferably between 15% and 17%.

These structures are also substantially similar to articulatedquadrilaterals or “Chebyshev guides” which are used in the “straight”portion when the side bars are crossed.

As already mentioned, the device 1 preferably defines a free conditionand at least one stress condition.

In the free condition, the device 1 is free with respect to seismicstresses and the first support plane 4 a, the second support plane 5 aand the third support plane 6 a are parallel to each other. In thiscondition, the device 1 is adapted to support the upper structure.

In the stress condition the device 1 is, instead, stressed by means of aseismic stress defining at least one displacement x.

The displacement x is for example arranged along the third support plane6 a and parallel to the connection plane 7 a so as to allow the device 1to be moved according to a displacement x.

In detail, the first support 1 undergoes the displacement x by theseismic stress on the ground 3 and, as a result, all the supports 5, 6arranged above are consequently moved.

Structurally, the device 1, described so far in terms of two-dimensionalmodel, may comprise a plurality of pairs of first and second bars 70,71.

Such first and second bars 70, 71 reciprocally coupled to other firstand second bars 70, 71 are preferably parallel to the latter andarranged along parallel and spaced connection planes 7 a.

In addition, the supports 4, 5, 6 may be formed or comprise a pluralityof different structural elements.

For example, the first support 4 may comprise a first support bar 41,the second support 5 may comprise a second support bar 51 and the thirdsupport may comprise a third support bar 61.

The support bars 41, 51, 61 preferably rigidly connect, in thisconfiguration, the hinges 40, 50, 60, respectively.

This configuration can be used for devices 1 that extend vertically in atwo-dimensional manner, i.e. mainly along the connection plane 7 a andwith two first bars 70 and two second bars 71.

In particular, the device 1 may comprise adjacent pairs of first bars 70and second bars 71 connected to adjacent pairs of support bars 41, 51,61.

In this case the hinges 40, 50, 60 comprise spacers adapted to connectthe pairs of support bars 41, 51, 61 and bars 70, 71 and the device 1 issubstantially made of two structures, as described in the previousconfiguration, adjacent and constrained in a mirror-like manner.

Alternatively, the device 1 may comprise a first support plate, a secondsupport plate and a third support plate.

In detail, the first support 4 may comprise the first support plate, thesecond support 5 may comprise the second support plate and the thirdsupport may comprise the third support plate.

The support plates are preferably coplanar with respect to the supportplanes 4 a, 5 a, 6 a respectively and are adapted to respectivelyconnect the hinges 40, 50, 60. Such support plates may further beconnected by two first bars 70 and two second bars 71, or by a pluralityof pairs of bars 70, 71.

Preferably, the device 1 is adapted to be used, as already mentioned,for anti-seismic foundations for building type structures.

In this case the seismic foundations comprise at least one device 1 andpart, typically of the structure 2.

The device 1 can thus be arranged between two structural portions 2 orbetween the ground and a structural portion 2, typically the base.

The foundations comprising the device 1 may further provide fordifferent configurations.

They may comprise a single device 1 or a plurality thereof.

For example, a foundation may comprise a plurality of devices 1 whereinall the respective third support planes 6 a are all coplanar.

Preferably, moreover, all the first support planes 4 a are alsocoplanar.

Such a configuration is, for example, shown in FIG. 5.

In addition, an anti-seismic foundation may comprise a plurality ofdevices 1 arranged consecutively in an overlapping manner and wherein,i.e., one of the third support planes 6 a is integral with a lowerportion, e.g., the ground 3, one of the first support planes 4 a isintegral with an upper portion, e.g., the structure 2, and the otherfirst support planes 4 a and third support planes 6 a are integral witheach other. In addition, preferably the devices 1 are not overlappedalong coplanar connection planes 7 a, but each of the devices 1 definesat least one free connection plane 7 a skewed with respect to theconnection planes 7 a of the other devices 1 so as to allow thefoundation to absorb a plurality of displacements x connected to seismicstresses in different directions along the connection planes 7 a, asshown in FIG. 6. For example, preferably, a foundation may comprise fouroverlapping devices 1 so as to realize a column in which each device 1defines a connection plane skewed with respect to the adjacent ones withinclination preferably equal to 45°. In this case, the devices 1 mayhave preferably octagonal perimeter edges.

In this way, a foundation is created that can absorb seismic stressesfrom the ground 3 with displacements x-within four different directions.

Even two overlapping devices 1 defining mutually perpendicularconnection planes 7 a can be sufficient to cushion all the coplanarforces, since the forces can always be separated along two perpendicularaxes.

The functioning of the device 1 described above in structural terms, isas follows. When it is in the free condition all the support planes 4 a,5 a, 6 a are mutually parallel and the bars 70, 71 preferably intersectat a point comprised in the geometric axis of the device 1.

When the device 1 passes from the free condition to the stress conditiondue to seismic stresses that impress displacements x on the firstsupport 4, the first support 4 is subjected to displacement x if it isparallel to the connection plane 7 a. When the first support 4 moves,and typically vibrates, the crossing point of the bars 70, 71 is offsetfrom the axis of the device 1 and the second support plane 5 a tiltscorrespondingly to the inclination of the second bars 71.

Similarly, the first bars 70 and the first support plane 4 a are tiltedwith respect to the second support plane 5 a.

If the first support plane 4 a and the third support plane 6 a areintegrally constrained respectively to an upper and lower portioncharacterized by sufficient values of moments of inertia, the limits ofwhich are readily detectable depending on the dimensions of the device 1from experimental tests, the first support 4 and the third support 6remain parallel during the movement of the device 1.

In this case, the support planes 4 a, 6 a remain parallel and only thesecond support plane 5 a is tilted with the bars 70, 71 which performopposite rotations. More specifically, when the second support plane 5 arotates, the support planes can only reciprocally translate along aplane parallel to the ground 3, or along a direction perpendicular tothe ground 3.

However, this latter movement is extremely limited, and negligible, forstresses with vibrations at low intensity or amplitude.

As a result, the device 1 permits a substantial “floating” effect to beobtained when a structure 2 is subjected to seismic stresses present onthe ground 3. The device 1 according to the invention achieves importantadvantages.

In fact, the device 1 allows the stresses deriving from seismicactivities to be absorbed in a dynamic and mechanical manner, i.e.without resorting to elements subject to deformations.

Consequently, the device 1 allows the movements and displacements ximpressed by seismic stresses to be absorbed not only due to therigidity of the constituent elements, but also thanks to the kinematicmechanisms included within the device 1.

In fact, in relation to the dimensions and configurations of the device1, or of the foundations that comprise it, it is possible to completelyabsorb the vibration modes of the seismic stresses through the relativemovement of the first support plane 4 a with respect to the thirdsupport plane 6 a.

This absorption takes place in a completely stable manner as the device1 tends to return to the free condition, when not stressed. Therefore,the free condition realized by the device 1 is a stable equilibriumcondition.

In conclusion, the device 1 allows the degrees of freedom of themovements to which the structure 2 is subjected to be reduced, forexample with respect to the ground 3 since it is not allowed to rotatearound an axis parallel to the first support plane 4 a.

Variations may be made to the invention described herein withoutdeparting from the scope of the inventive concept defined in the claims.

For example, the supports 4, 5, 6 can be constrained together by meansof elastic elements and/or dampers adapted to control and, if necessary,vary the dynamic response of the device 1 to seismic stresses.

Examples of embodiments of this type are shown in FIG. 3 and FIG. 4.

Preferably, such elastic elements may be common springs and the dampersmay be of the hydraulic type and configurations may be provided for inwhich, for example, the first hinges 40 are connected by said elasticelements and/or dampers to the second hinges 50 and, in turn, the secondhinges 50 may be connected to the third hinges 60.

These may be either of the passive type or active type. The device 1could also actively compensate seismic movements.

In said sphere all the details may be replaced with equivalent elementsand the materials, shapes and dimensions may be as desired.

1. An anti-seismic device for seismically isolating a structure withrespect to the ground wherein it comprises: a first support defining afirst support plane integrally connectable to an upper portion, forexample to said structure, and comprising at least two first hingesdefining a first constant reciprocal distance (d′), a second supportdefining a second support plane comprising at least two second hingesdefining a second constant reciprocal distance (d″), a third supportdefining a third support plane integrally connectable to a lowerportion, for example to said ground, and comprising at least two thirdhinges defining a third constant reciprocal distance (d′″), andconnection means defining a connection plane perpendicular to said thirdsupport plane and comprising at least: two first rigid bars, eachdefining a first non-deformable direction of connection and adapted toconstrain said first support and said second support, and two secondrigid bars, each defining a second non-deformable direction ofconnection and adapted to constrain said second support and said thirdsupport, said first bars each being transiently constrained to one ofsaid first hinges and one of said second hinges so that said firstdirections of connection of said first bars cross in said connectionplane, and said second bars each being transiently constrained to one ofsaid second hinges and one of said third hinges so that said seconddirections of connection of said second bars cross in said connectionplane.
 2. The device according to claim 1, wherein said first distance(d′) and said third distance (d′″) are congruent and said seconddistance (d″) is less than said first and third distances (d′, d′″). 3.The device according to claim 1, wherein said second distance (d″) is atleast 3% less than said third distance (d′″).
 4. The device according toclaim 1, wherein said supports and said connection means define at leasttwo superimposed “Chebyshev guides”.
 5. The device according to claim 1,comprising a plurality of pairs of said first and second bars, which areparallel to each other along parallel and spaced connection planes. 6.The device according to claim 1, wherein said supports each respectivelycomprise at least one support rod respectively adapted to rigidlyconnect said hinges.
 7. The device according to claim 1, wherein saidsupports each respectively comprise at least one support platerespectively coplanar with said support planes and respectively adaptedto rigidly connect said hinges.
 8. The device according to claim 1,comprising two pairs of two second hinges.
 9. Earthquake-resistantfoundations of a structure comprising a device according to claim 1 andat least part of said structure, said structure being a building-typestructure.
 10. The earthquake-resistant foundations according to claim1, comprising a plurality of said devices, wherein said respective thirdsupport planes are all coplanar.
 11. The earthquake-resistantfoundations according to claim 1, comprising a plurality of saiddevices, wherein said devices are arranged consecutively overlapping,one of said third support planes being integral with a lower portion,one of said first support planes being integral with an upper portion,said other first support planes and third support planes being integralwith each other, and each of said devices defining at least one saidconnection plane which is skewed with respect to said connection planesof said other devices so as to allow said foundations to absorb aplurality of seismic stresses in different directions along saidconnection planes.